1. Field of the Invention
The present relates to a developing device using a two-ingredient type developer and an image forming apparatus including the same.
2. Description of the Background Art
It is a common practice with a copier, facsimile apparatus, printer or similar image forming apparatus to develop a latent image formed on a photoconductive element or image carrier with toner or similar developer. A two-ingredient type developer made up of toner and carrier is one of developers customarily used for development. The prerequisite with development using the two-ingredient type developer is that fresh toner be replenished in accordance with consumption in order to maintain the toner content of the developer constant. Various schemes have heretofore been proposed for meeting this prerequisite, as will be described hereinafter.
Japanese Patent Laid-Open Publication Nos. 7-219324, 7-219329 and 9-15957, for example, disclose a toner bottle, toner tank or similar hard toner container storing fresh toner and located in the toner replenishing section of a developing device and an arrangement for delivering the toner from the toner container to a developing case by suction or vacuum. Japanese Patent Laid-Open Publication Nos. 11-282238, 12-47464, 12-14789, 12-351445 and 12-356898, for example, teach toner replenishment using a bag as a toner container. Japanese Patent Laid-Open Publication No. 9-244372, for example, proposes a mechanism in which a toner container performs asymmetric reciprocating movement so as to cause toner to flow out via an outlet formed in the bottom of the container due to the resulting vibration. Japanese Patent Laid-Open Publication Nos. 7-20701, 7-20703 and 7-114260, for example, each disclose a toner container located in the vicinity of a developing case and configured to replenish fresh toner to the case mainly by gravity.
While fresh toner is replenished in accordance with consumption in order to maintain the toner content of the developer constant, the carrier is, in many cases, used without regard to the replenishment of fresh carrier and therefore deteriorated due to repeated agitation. The deterioration of the carrier includes fatigue ascribable to the wear of the carrier itself, damage to a coating layer used to increase the charging ability, and toner filming, i.e., adhesion of toner to the carrier. The deterioration of the carrier is apt to lower the charging ability of toner.
To obviate the fall of the charging ability of toner ascribable to the deterioration of carrier, there has been proposed a developer replacement system called a trickle development system. The trickle development system replenishes a fresh carrier independently of the replenishment of toner while discharging the resulting excess part of a developer, thereby replacing the developer containing deteriorated carrier with a fresh developer.
As for the trickle development system, Japanese Patent Publication No. 2-21591 and Japanese Patent Laid-Open Publication Nos. 9-166912, 9-218575 and 9-244376, for example, each propose to replenish a toner and carrier mixture or so-called premixed developer. Japanese Patent Laid-Open Publication No. 9-204105, 9-251235 and 9-269644, for example, each teach a system configured to replenish toner and carrier to a developing device individually while collecting excess part of the developer from a developing case. Japanese Patent Laid-Open Publication Nos. 10-63074 and 10-63075, for example, disclose a system configured to control the replenishment of toner in accordance with the consumption of toner, the replenishment of carrier in accordance with the amount of toner replenished, and the discharge of the developer. Japanese Patent Lad-Open Publication Nos. 7-234575 and 2001-194860, for example, each propose a system in which a single carrier replenishing section is shared by a plurality of developing devices. Japanese Patent Laid-Open Publication Nos. 11-143196 and 11-272075, for example, each disclose a system configured to feed toner and carrier to a developing case while controlling their mixture ratio.
However, the conventional constructions and systems described above have the following problems left unsolved. As for the replenishment of toner, a toner container is, in many cases, implemented as a hard bottle having a substantial volume. The number of such hard bottles that can be collected for a unit capacity is limited, resulting in high collection cost. Although a contractible bag-like toner container has been proposed, it lacks an implementation for delivering substantially the entire toner stored therein, so that much toner is left in the container and increases consumption cost.
Today, a screw auger is extensively used for conveying fresh toner to be replenished. However, a screw auger must be configured integrally with or located in the vicinity of a developing device or a toner container due to its structure, complicating the entire structure and thereby increasing cost. Further, not only a portion to be maintained but also the entire subassembly must be dismounted. Such maintenance is difficult for the user to perform.
To replace a developer to be replenished, it has been customary to disassemble a developing device, remove a developer container, refill the container, and again assemble the developing device. Such replacement is difficult for the user to perform and, in many cases, relies on a service person, resulting in down-time and forcing the user to bear extra expense. Although the trickle development system reduces the frequency of replacement that needs the above procedure, it cannot solve the problems relating to collection and conveyance because a fresh toner container and a fresh carrier container themselves are the same as in the case of toner replenishment described above.
In the trickle development system, the mixture ratio of toner and carrier to be replenished remains constant. Therefore, when the developer is consumed in a large amount, e.g., when images with a large size or high density are continuously formed, the carrier is replenished along with the toner that is replenished in accordance with a change in toner content. Stated another way, the carrier is replaced with fresh carrier without regard to the life of the carrier present in the developer. This is wasteful and forces the user to bear high maintenance cost.
Conversely, when images with low density are continuously formed, the carrier is not replaced because the consumption of toner decreases. Consequently, the carrier of the developer is simply, repeatedly charged by agitation and continuously used even when it is deteriorated. Therefore, when the carrier is replaced on the basis of a change in the toner content of the developer, it is likely that the life of the carrier differs from the actual condition, resulting in wasteful replenishment, an increase in cost, and the fall of charging ability of toner. Particularly, when the toner is consumed little and when the life of carrier is determined to have ended, it is necessary, in the worst case, to replace the entire developer present in the developing device or the developing device itself, interrupting image formation over a long period of time.